What Is the Purpose of a Flywheel in a Car?

by Mike Southern

Every car has a flywheel. Flywheels are heavy metal disks, between 12 and 15 inches in diameter, with gear teeth cut into its circumference. They are attached to the rear of the crankshaft, between the engine and the transmission. Sometimes they are called flex plates when connected to an automatic transmission, and in this case they are lighter than the flywheels used with manual transmissions.

Starting the Engine

The gear teeth cut into the flywheel’s circumference help with starting the engine. The engine’s starter motor has a small gear (called a Bendix gear) that mates up to the flywheel when you turn the key. The Bendix gear / starter motor combination spins the flywheel, turning the crankshaft and beginning the compression cycle necessary to start the engine. After the engine starts, the Bendix gear withdraws so the flywheel can spin freely.

Smoothing Engine Speed

Once the engine has started, the crankshaft converts the up-and-down motion of the pistons into rotary motion. However, this motion is jerky because power is generated only twice (for a four-cylinder engine) or four times (for an eight-cylinder) during one engine revolution. The mass of the flywheel uses inertia to keep the engine crankshaft turning between each of these piston firings, so the rotation speed of the crankshaft is kept constant and the engine runs smoothly.

Balancing the Engine

Because the pistons are offset from the center of the crankshaft, the engine vibrates and wobbles as each piston fires at a different angle. The heavy weight of the flywheel resists this side-to-side motion, helping to stabilize and balance the engine on its mounts and reduce vibration throughout the vehicle.

Reduce Drivetrain Stress

By stabilizing the engine’s movement and smoothing out its speed, the flywheel reduces wear and tear on other drivetrain components. Although the engine and the transmission shafts are parallel to each other, the attachments between the transmission and the axle are not; the driveshaft uses universal joints that constantly change angles as they turn. Flywheels help reduce the wear on these joints.

How Weight Helps

Flywheel weight is one of the adjustments custom engine builders use to tailor their engine’s performance for specific uses. Heavier flywheels help engines that work under loads that may cause the engine to bog down; cars that consistently pull heavy trailers might benefit from a larger flywheel. Engines that run at high speeds—like race car engines—benefit from a lighter flywheel to improve acceleration at speed; this can make it harder to keep the engine idling smoothly and more difficult to accelerate from a dead stop. This is why racers need their pit crews to help start their cars by pushing them.

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